Communication control method, radio communication system, radio base station and user terminal

ABSTRACT

The present invention is designed to reduce the deterioration of communication quality even when different DL/UL configurations are applied between neighboring transmitting/receiving points (radio base stations). A radio communication method in a radio communication system, in which a plurality of radio base stations that communicate with a user terminal by means of time division duplexing each change the DL/UL configuration independently, includes the steps in which each radio base station generates a control signal that designates the number and positions of subframes to be allocated for DL and/or UL transmission among subframes constituting a radio frame, and transmits the control signal to the user terminal on a downlink control channel of a specific subframe constituting the radio frame.

TECHNICAL FIELD

The present invention relates to a communication control method, a radiocommunication system, a radio base station and a user terminal that areapplicable to cellular systems and so on.

BACKGROUND ART

In a UMTS (Universal Mobile Telecommunications System) network, attemptsare made to optimize features of the system, which are based on W-CDMA(Wideband Code Division Multiple Access), by adopting HSDPA (High SpeedDownlink Packet Access) and HSUPA (High Speed Uplink Packet Access), forthe purposes of improving spectral efficiency and improving the datarates. With this UMTS network, long-term evolution (LTE) is under studyfor the purposes of further increasing high-speed data rates, providinglow delay, and so on (non-patent literature 1).

In a third-generation system, it is possible to achieve a transmissionrate of maximum approximately 2 Mbps on the downlink by using a fixedband of approximately 5 MHz. Meanwhile, in an LTE system, it is possibleto achieve a transmission rate of about maximum 300 Mbps on the downlinkand about 75 Mbps on the uplink by using a variable band, which rangesfrom 1.4 MHz to 20 MHz. Also, in the UMTS network, successor systems ofthe LTE system (referred to as, for example, “LTE-Advanced” or “LTEenhancement” (hereinafter referred to as “LTE-A”)) are under study forthe purpose of achieving further broadbandization and increased speed.

In radio communication, as uplink (UL) and downlink (DL) duplexingmethods, there are frequency division duplexing (FDD), which dividesbetween the uplink and the downlink based on frequency, and timedivision duplexing (TDD), which divides between the uplink and thedownlink based on time. In the event of TDD, the same frequency isapplied to uplink and downlink transmission, so that the uplink and thedownlink are divided based on time and transmitted from one transmittingpoint. Since the same frequency is used between the uplink and thedownlink, a transmitting point (radio base station) and a user terminalboth have to switch between transmission and reception alternately.

Also, in TDD in the LTE system, frame configurations (transmissionratios between uplink subframes and downlink subframes (DL/ULconfigurations)) to support a plurality of different types ofasymmetrical uplink/downlink resource allocation are defined (see FIG.1). In the LTE system, as shown in FIG. 1, seven frame configurations,namely DL/UL configurations 0 to 6, are defined, where subframes #0 and#5 are allocated to the downlink and subframe #2 is allocated to theuplink. Also, to prevent interference between transmitting points (orbetween cells), the same DL/UL configuration is applied betweenneighboring transmitting points.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: 3GPP, TR25.912 (V7.1.0), “Feasibility Study    for Evolved UTRA and UTRAN,” September 2006

SUMMARY OF INVENTION Technical Problem

However, in TDD in the LTE-A system, in order to allow efficient use ofradio resources, a study is in progress to change the transmission ratioof DL and UL dynamically or semi-statically in the time domain, pertransmitting/receiving point—that is, change the DL/UL configuration toapply on a per transmitting/receiving point basis. When different DL/ULconfigurations are applied between neighboring transmitting/receivingpoints, cases might occur where, in the same time region/frequencyregion, a DL subframe and a UL subframe are transmitted at the same timebetween neighboring transmitting/receiving points (cases where an uplinksignal and a downlink signal are transmitted at the same time).

In this case, depending on the location and transmission power of eachtransmitting/receiving point (or user terminal) and so on, there is athreat that interference is produced between transmitting/receivingpoints and between user terminals, and the performance of communicationquality deteriorates.

The present invention has been made in view of the above, and it istherefore an object of the present invention to provide communicationcontrol method, a radio communication system, a radio base station and auser terminal that can reduce the deterioration of communication qualityeven when different DL/UL configurations are applied between neighboringtransmitting/receiving points (radio base stations).

Solution to Problem

The communication control method of the present invention is acommunication control method in a radio communication system in which aplurality of radio base stations that communicate with a user terminalby means of time division duplexing each change a DL/UL configurationindependently, and this communication control method includes the stepsin which each radio base station generates a control signal thatdesignates the number and positions of subframes to be allocated for DLand/or UL transmission among subframes constituting a radio frame, andtransmits the control signal to the user terminal on a downlink controlchannel of a specific subframe constituting the radio frame.

Advantageous Effects of Invention

According to the present invention, it is possible to reduce the impactof interference even when different DL/UL configurations are appliedbetween neighboring transmitting/receiving points (radio base stations).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram to explain examples of DL/UL configurations in TDD;

FIG. 2 provides diagram to show an example of a radio communicationsystem where different DL/UL configurations are applied betweenneighboring radio base stations;

FIG. 3 is a diagram to explain an example of a radio communicationsystem according to the present embodiment;

FIG. 4 is a diagram to explain an example of a radio frame for reportingDL/UL subframe information in a radio communication system according tothe present embodiment;

FIG. 5 is a diagram to explain an example of a DL subframe informationmanagement table in a radio communication system according to thepresent embodiment;

FIG. 6 is a diagram to explain an example of a UL subframe informationmanagement table in a radio communication system according to thepresent embodiment;

FIG. 7 is a diagram to explain example of a DL/UL subframe informationmanagement table in a radio communication system according to thepresent embodiment;

FIG. 8 is an example of a sequence diagram to show a communicationcontrol method in a radio communication system according to the presentembodiment;

FIG. 9 is a diagram to explain an overall configuration of a radio basestation;

FIG. 10 is a functional block diagram corresponding to a basebandprocessing section of a radio base station;

FIG. 11 is a diagram to explain an overall configuration of a userterminal; and

FIG. 12 is a functional block diagram corresponding to a basebandprocessing section of a user terminal.

DESCRIPTION OF EMBODIMENTS

First, an example of a radio communication system where the presentembodiment is applied will be described with reference to FIG. 2A. Theradio communication system shown in FIG. 2A is formed to include aplurality of transmitting/receiving points (here, radio base stations #1and #2), and user terminals #1 and #2 that communicate with radio basestations #1 and #2.

Also, in this radio communication system, radio communication betweenradio base station #1 and user terminal #1 (between radio base station#2 and user terminal #2) is conducted by means of time divisionduplexing (TDD). That is, in radio base stations #1 and #2, the samefrequency regions are applied to DL and UL transmission, andtransmission from each radio base station is conducted by dividingbetween DL and UL in the time domain.

In this case, as noted earlier, if different DL/UL configurations areapplied between neighboring radio base stations #1 and #2, there is athreat of deterioration of communication quality performance due tointerference between radio base stations #1 and #2 and interferencebetween user terminals #1 and #2 in predetermined subframes.

For example, when, as shown in FIG. 2B, radio base station #1 adoptsDL/UL configuration 1 and radio base station #2 adopts DL/ULconfiguration 2 in a given period (here, in one radio frame), insubframes #3 and #8, radio base station #1 carries out UL transmissionwhile radio base station #2 carries out DL transmission. That is, in thesame time regions and in the same frequency regions, downlink signalsare transmitted from radio base station #2 to user terminal #2, whileuplink signals are transmitted from user terminal #1 to radio basestation #1.

In this case, the downlink signals that are transmitted from userterminal #2 to radio base station #2 become interference against theuplink signals transmitted from user terminal #1 to radio base station#1 (interference between radio base stations #1 and #2). Also, theuplink signals that are transmitted from user terminal #1 to radio basestation #1 become interference against the downlink signals transmittedfrom radio base station #2 to user terminal #2 (interference betweenuser terminals #1 and #2). As a result of this, there is a threat thatthe received quality of radio base station #1 and the received qualityof user terminal #2 lower in subframes #3 and #8.

So, the present inventors have found out that, by allowing each radiobase station to adequately control the DL/UL configuration taking intoaccount the level of interference between transmitting/receiving points(radio base stations) or between user terminals, it is possible toreduce the deterioration of communication quality even when differentDL/UL configurations are applied between neighboringtransmitting/receiving points.

Now, the present embodiment will be described below in detail withreference to the accompanying drawings. Note that an example with twotransmitting/receiving points (radio base stations) will be shown in thefollowing description for ease of explanation. However, the number oftransmitting/receiving points which the present invention can employ isnot limited to this and can be changed as appropriate.

FIG. 3A shows an example of a radio communication system according tothe present embodiment. The radio communication system of FIG. 3 isformed to include a plurality of transmitting/receiving points (here,radio base stations #1 and #2) and user terminals #1 and #2 that areconnect to radio base stations #1 and #2, respectively. Note that radiobase stations #1 and #2 are able to communicate information with eachother through wire connection such as optical fiber and so on, orthrough wireless connection.

To radio communication between radio base station #1 and user terminal#1 and between radio base station #2 and user terminal #2, time divisionduplexing (TDD) is applied. Also, in the radio communication systemshown in FIG. 3, radio base stations #1 and #2 separately(independently) change and control the DL/UL transmission ratio (DL/ULconfiguration) in the time domain. In this case, radio base stations #1and #2 each select an arbitrary DL/UL configuration from DL/ULconfigurations that are provided in advance. Note that the details ofthe DL/UL configurations selected in radio base stations #1 and #2 willbe described later.

When different DL/UL configurations are applied between neighboringradio base stations #1 and #2, a downlink signal and an uplink signalare transmitted in the same frequency region/the same time regionbetween the neighboring radio base stations (or between cells). Forexample, as shown in FIG. 3, for radio base station #1 that receives anuplink signal from user terminal #1, a downlink signal that istransmitted from another radio base station #2 to user terminal #2becomes an interference signal. Because such interference signals exist,the overall communication quality of the radio communication systemdeteriorates.

So, with the radio communication system according to the presentembodiment, each radio base station communicates with serving userterminals by dynamically switching the DL/UL configuration to adopt on aper radio frame basis. Based on the level of interference to receivefrom other radio base stations, each radio base station requests changesof the DL/UL configuration to other radio base stations. By this means,each radio communication adopts a common DL/UL configuration with otherradio base stations and communicates with serving user terminals. As aresult of this, it is possible to reduce the impact of interferencebetween radio base stations and between user terminals, and reduce thedeterioration of the overall communication quality of the radiocommunication system.

Upon switching the DL/UL configuration dynamically, each radio basestation reports information related to subframes for DL and/or ULtransmission (hereinafter referred to as “DL/UL subframe information”)by means of, for example, a downlink control channel (PDCCH, ePDCCH)that is allocated to a specific subframe in a radio frame. This DL/ULsubframe information includes the number and positions of subframes forDL and/or UL transmission. By demodulating this downlink controlchannel, the user terminal can specify the subframes where the downlinkdata channel (PDSCH) for that user terminal is allocated, the subframesto allocate an uplink control channel (PUCCH) and an uplink data channel(PUSCH) to.

Note that the PDCCH (downlink control channel) is a downlink controlchannel that is placed over a predetermined number of OFDM symbols (oneto three OFDM symbols) from the top of a subframe, and is a controlchannel that is time-division-multiplexed with the PDSCH (downlinkshared data channel). Also, the ePDCCH (also referred to as “enhanceddownlink control channel,” “E-PDCCH,” “Enhanced PDCCH,” “FDM-typePDCCH,” “UE-PDCCH” and so on) is a control channel that is placed to befrequency-division-multiplexed with the PDSCH.

FIG. 4 is a diagram to explain an example of a radio frame for reportingDL/UL subframe information in the radio communication system accordingto the present embodiment. For example, DL/UL subframe information maybe reported in the downlink control channel (PDCCH, ePDCCH) that isallocated to the top subframe (subframe #0) of a radio frame. In thisway, by setting in advance a specific subframe in a radio frame as thesubframe to allocate DL/UL subframe information to, it is not necessaryto report the specific subframe to a user terminal in advance. By thismeans, the control upon reporting DL/UL subframe information from eachradio base station to user terminals can be simplified.

Note that the subframe to allocate DL/UL subframe information to is byno means limited to subframe #0 at the top. For example, DL/UL subframeinformation may be allocated to subframes other than top subframe #0 aswell. Also, it is possible to report the subframe where DL/UL subframeinformation is allocated to user terminals through broadcast signals,higher layer signaling (for example, RRC signaling) and so on. In thisway, by reporting a specific subframe where DL/UL subframe informationis allocated by broadcast signals and RRC signaling, it is possible toflexibly change the subframe to allocate DL/UL subframe information to,depending on the communication environment.

In the radio communication system according to the present embodiment,the number and positions of subframes for DL and/or UL transmissionincluded in DL/UL subframe information are determined in advance. Forexample, the number and positions of subframes for DL and/or ULtransmission are managed by means of a table. In this management table,subframe positions are defined in association with numbers of subframesfor DL and/or UL transmission. In this way, by determining the numberand positions of subframes for DL and/or UL transmission in advance, itis not necessary to determine the number and positions of subframes tobe allocated for DL and/or UL transmission every time a downlink controlchannel signal (control signal) is generated. By this means, it becomespossible to simplify the process upon generating downlink controlchannel signals. Also, it is possible to simplify the control whenselecting a DL/UL configuration that is common between radio basestations.

FIG. 5 and FIG. 6 are diagrams to explain examples of DL/UL subframeinformation management tables in the radio communication systemaccording to the present embodiment. FIG. 5 shows an example of a DLsubframe information management table. FIG. 6 shows an example of a ULsubframe information management table. Note that the configurations ofthe DL/UL subframe information management tables are not limited to thecontents shown in FIG. 5 and FIG. 6.

As shown in FIG. 5 and FIG. 6, in the management tables, DL/ULconfiguration pattern indices (configuration pattern indices), numbersof subframes for DL or UL transmission, and the indices of subframesthat are allocated as subframes for DL or UL transmission (that is, thepositions of subframes for DL or UL transmission). Note that, in bothmanagement tables, subframe #1 is designated a special subframe.Consequently, subframe #1 is never allocated as a subframe for DL or ULtransmission.

As shown in FIG. 5, in the DL subframe information management table,when the number of subframes is one (in the event of pattern index #0),a subframe for DL transmission is allocated to subframe #0 alone. Also,when the number of subframes is two (in the event of pattern index #1),subframes for DL transmission are allocated to subframes #0 and #3.Then, as the number of subframes increases one by one, the number ofsubframes where subframes for DL transmission are allocated increasesone by one, from subframe #4 onward. Note that when the number ofsubframes is eight (in the event of pattern index #7), subframes for DLtransmission are allocated to all the subframes except for subframe #1and #2.

Meanwhile, as shown in FIG. 6, in the UL subframe information managementtable, when the number of subframes is one (in the event of patternindex #10), a subframe for UL transmission is allocated to subframe #2alone. Also, when the number of subframes is two (in the event ofpattern index #11), subframes for UL transmission are allocated tosubframes #2 and #3. Then, as the number of subframes increases one byone, the number of subframe where subframe for UL transmission areallocated increase one by one, from subframe #4 onward. Note that, whenthe number of subframes is eight (in the event of pattern index #17),subframe for UL transmission are allocated to all the subframes exceptfor subframe #0 and #1.

FIG. 7 shows an example of a management table to manage both DL subframeinformation and UL subframe information. Note that FIG. 7 illustrates acase where three or fewer pieces of UL subframe information areallocated. In the management table shown in FIG. 7, the number ofsubframes for DL transmission and the number of subframes for ULtransmission are defined as the number of subframes. Also, in themanagement table shown in FIG. 7, subframes for DL transmission areallocated in the same manner as in the DL subframe informationmanagement table shown in FIG. 5, and subframes for UL transmission areallocated to the other subframes. When the numbers of subframes for DLtransmission and subframes for UL transmission are one and three (in theevent of pattern index #20), respectively, a subframe for DLtransmission is allocated to subframe #0 alone, and subframes for ULtransmission are allocated to subframes #2 to #4. Meanwhile, when thenumbers of subframes for DL transmission and subframes for ULtransmission are seven and two (in the event of pattern index #26),respectively, subframes for DL transmission are allocated to subframes#0 and #3 to #8, and subframes for UL transmission are allocated tosubframes #2 and #9.

These management tables are managed in each radio base station. That is,each radio base station shares these management tables. With the exampleshown in FIG. 7, a DL/UL subframe information management table in whichthree or fewer pieces of UL subframe information are allocated is shown.In addition to this, in each radio base station, a DL/UL subframeinformation management table, in which three or other numbers of piecesof UL subframe information are allocated, is also shared.

Each radio base station separately (independently) changes and controlsthe DL/UL configuration in the time domain. In this case, each radiobase station switches the DL/UL configuration dynamically with referenceto the above management tables. For example, when interference isproduced between transmitting/receiving points (radio base station), aDL/UL configuration that is common with other radio base stations isselected. Then, each radio base station reports DL/UL subframeinformation that corresponds to the selected DL/UL configuration to theserving user terminals through a downlink control channel. To be morespecific, each radio base station transmits a control signal todesignate DL/UL subframe information to the serving user terminalsthrough the downlink control channel.

The number and positions of subframes for DL transmission correspondingto the DL/UL configuration that is selected may be included, forexample, in a DL assignments in the downlink control information (DCI)that is transmitted in the downlink control channel. In this way, bydesignating the number and positions of subframes for DL transmission ina DL assignment within DCI, it is possible to report the number andpositions of subframes for DL transmission to user terminals withouthaving to make significant changes to conventional specifications.

Meanwhile, the number and positions of subframes for UL transmissioncorresponding to the DL/UL configuration that is selected may beincluded, for example, in a UL grant in downlink control information(DCI) that is transmitted in the downlink control channel. In this way,by designating the number and positions of subframes for UL transmissionin a UL grant within DCI, it is possible to report the number andpositions of subframes for UL transmission to user terminals withouthaving to make significant changes to conventional specifications.

For example, when the DL/UL configuration of pattern index #2 in themanagement table shown in FIG. 5 is selected, each radio base stationincludes the number of subframes (three) and their positions (subframes#0, #3 and #4) in the DL assignment in downlink control information(DCI). Also, when the DL/UL configuration of pattern index #12 in themanagement table shown in FIG. 6 is selected, each radio base stationincludes the number of subframes (three) and their positions (subframes#2, #3 and #4) in the UL grant in downlink control information (DCI).Furthermore, when the DL/UL configuration of pattern index #22 in themanagement table shown in FIG. 7 is selected, each radio base stationincludes the number of subframes (three) and their positions (subframes#0, #3 and #4) in the DL assignment in downlink control information(DCI), and includes the number of subframes (three) and their positions(subframes #2, #5 and #6) in the UL grant.

Now, an example of a sequence of a communication control method in theradio communication system according to the present embodiment will bedescribed below with reference to FIG. 8. Here, a case will be describedwhere radio base station #1 shown in FIG. 3 reports a DL/ULconfiguration change request (hereinafter referred to simply as “changerequest”) to another radio base station #2 based on the level ofinterference from the other radio base station #2. Note that, although acase will be shown with the following description where radio basestation #1 (the interfered station in FIG. 3) transmits a change requestsignal to another radio base station #2 (the interfering station in FIG.3), the other radio base station #2 is also able to perform the sameprocesses as those by radio base station #1.

First, radio base station #1 measures the level of interference fromanother radio base station #2 (step S101). Note that the level ofinterference according to the present embodiment refers to, for example,path loss, penetration loss, antenna gain and so on. For example, thepath loss of an uplink channel (between radio base station #1 and userterminal #1) may be measured. Note that when there is a plurality ofother radio base stations, the resulting level of interference is thetotal value of the levels of interference from this plurality of otherradio base stations.

Also, in step S101, radio base station #1 determines whether or not thelevel of interference that is measured is greater than a predeterminedreference value (threshold value #1) regarding the level ofinterference. Threshold value #1 is the reference value for determiningthe impact of interference from the other radio base station #2 to radiobase station #1. Radio base station #1 determines whether or not toissue a DL/UL configuration change request based on this threshold value#1.

Note that the predetermined reference value (threshold value #1) may beshared between radio base stations or may differ between radio basestations. Also, the predetermined reference value (threshold value #1)may be configured to be reported to radio base station #1 by means ofbroadcast signals, higher layer signaling (for example, RRC signaling)and so on. Also, the predetermined reference value (threshold value #1)may be reported to radio base station #1 via X2 signaling, optical fiberand so on. Besides, it is also possible to memorize threshold value #1in a memory section of radio base station #1 in advance, so that radiobase station #1 can apply threshold value #1 that is memorized.

When the level of interference from the other radio base station #2 isgreater than a predetermined reference value (threshold value #1),downlink signals that are transmitted from the other radio base station#2 have impact on radio base station #1, which receives uplink signalstransmitted from user terminal #1. In this case, radio base station #1request a change of the DL/UL configuration to the other radio basestation #2 (step S102).

To be more specific, radio base station #1 transmits a change requestsignal to request a change of the DL/UL configuration to the other radiobase station #2 so that the other radio base station #2 adopts the sameDL/UL configuration as in radio base station #1. Note that this changerequest signal includes the pattern index of the DL/UL configurationafter the change (that is, the same DL/UL configuration as in radio basestation #1).

The other radio base station #2, having received the change requestsignal from radio base station #1, selects the DL/UL configurationcorresponding to the DL/UL configuration pattern index included in thechange request signal (step S103). Also, in step S103, radio basestation #2 changes the presently adopted DL/UL configuration to theDL/UL configuration that is selected.

By this means, the DL/UL configuration that is adopted in radio basestation #2 is changed to the same DL/UL configuration as the DL/ULconfiguration adopted in radio base station #1. Note that, in the DL/ULconfiguration after the change, common DL transmission or ULtransmission is carried out in the same time regions and the samefrequency regions. Consequently, it is possible to reduce theinterference produced between radio base stations #1 and #2 and theinterference between user terminals #1 and #2.

Then, radio base stations #1 and #2 each generate a control signal thatdesignates DL/UL subframe information corresponding to the DL/ULconfiguration to be adopted. Then, the generated control signals aretransmitted to serving user terminals #1 and #2 through the downlinkcontrol channel of specific sub frames in a radio frame. User terminals#1 and #2 receive these control signals, and analyze the DL/UL subframeinformation that is designated in the control signals. After that, radiobase stations #1 and #2 each adopt a DL/UL configuration that reflectsthe change request reported from the other radio base station, andconduct radio communication with serving user terminals #1 and #2 (stepsS104 a and 104 b).

With the radio communication system according to the present embodiment,among subframes that constitute a radio frame, a control signal todesignate DL/UL subframe information is transmitted on a downlinkcontrol channel of a specific subframe (for example, the top subframe)in that radio frame. By this means, it is possible to change the DL/ULconfiguration to adopt, dynamically, on a per radio frame basis. As aresult of this, even when different DL/UL configurations are adoptedbetween neighboring transmitting/receiving points (radio base stations),it is still possible to reduce the impact of interference between radiobase stations and between user terminals by adequately switching andcontrolling the DL/UL configuration, and reduce the deterioration ofcommunication quality.

In particular, with the radio communication system according to thepresent embodiment, when a change request signal to request a change ofthe DL/UL configuration is received from another radio base station, acontrol signal to designate DL/UL subframe information is generated.Consequently, it is possible to switch the DL/UL configuration only whenit is necessary to change the DL/UL configuration. As a result of this,it is possible to apply different DL/UL configurations betweenneighboring transmitting/receiving points (radio base stations) andstill reduce the impact of interference between radio base stations andbetween user terminals as needed.

Now, the sequence shown in FIG. 8 will be described using specificexamples. Assume that, for example, in a given period (here, one radioframe), radio base station #1 adopts the DL/UL configuration of patternindex #21 shown in FIG. 7, and radio base station #2 adopts the DL/ULconfiguration of pattern index #22 shown in FIG. 7. In this case, insubframe #4, radio base station #2 carries out DL transmission, andradio base station #1 carries out UL transmission. That is, in the sametime regions and in the same frequency regions, downlink signals aretransmitted from radio base station #2 to user terminal #2, while uplinksignals are transmitted from user terminal #1 to radio base station #1.

The downlink signals that are transmitted from radio base station #2 touser terminal #2 become interference against the uplink signalstransmitted from user terminal #1 to radio base station #1 (interferencebetween radio base stations #1 and #2). Also, the uplink signals thatare transmitted from user terminal #1 to radio base station #1 becomeinterference against the downlink signals transmitted from radio basestation #2 to user terminal #2 (interference between user terminals #1and #2). Radio base station #1 measures these interference levels (stepS101). Assume here that the levels of interference that arise here aregreater than a predetermined reference value.

In this case, radio base station #1 transmits a change request signal torequest a change of the DL/UL configuration to the other radio basestation #2 so that the other radio base station #2 adopts the same DL/ULconfiguration as in radio base station #1 (step S102). This changerequest signal includes pattern index #21 of the DL/UL configurationafter the change (that is, the same DL/UL configuration as in radio basestation #1).

The other radio base station #2 selects the DL/UL configurationcorresponding to DL/UL configuration pattern index #21 included in thischange request signal (step S103). Then, the other radio base station #2adopts the selected DL/UL configuration as the DL/UL configuration thenand later. In this case, the DL/UL configurations to be adopted in radiobase station #1 and radio base station #2 both become the DL/ULconfiguration of pattern index #21 shown in FIG. 7. By this means, insubframe #4, which is the source of interference, radio base station #1and radio base station #2 carry out UL transmission. Consequently, theinterference that is produced between radio base stations #1 and #2 andthe interference produced between user terminals #1 and #2 are reduced.

Then, radio base stations #1 and #2 each generate a control signal todesignate DL/UL subframe information that corresponds to the DL/ULconfiguration of pattern index #21. Then, the generated control signalsare transmitted to serving user terminals #1 and #2 in the downlinkcontrol channel of a specific sub frame in a radio frame. User terminals#1 and #2 receive these control signals and analyze the DL/UL subframeinformation designated by the control signals. After that, radio basestations #1 and #2 each adopt the DL/UL configuration of pattern index#21 shown in FIG. 7, and conduct radio communication with serving userterminals #1 and #2 (steps S104 a and 104 b).

An overall configuration of a radio base station 20 according to thepresent embodiment will be described with reference to FIG. 9. As shownin FIG. 9, the radio base station 20 has transmitting/receiving antennas201, amplifying sections 202, transmitting/receiving sections(transmitting sections/receiving sections) 203, a baseband signalprocessing section 204, a call processing section 205, and atransmission path interface 206. Transmission data that is transmittedfrom the radio base station 20 to user terminals on the downlink isinput from the higher station apparatus, into the baseband signalprocessing section 204, via the transmission path interface 206.

In the baseband signal processing section 204, a downlink data channelsignal is subjected to a PDCP layer process, division and coupling oftransmission data, RLC (Radio link Control) layer transmission processessuch as an RLC retransmission control transmission process, MAC (MediumAccess Control) retransmission control, including, for example, an HARQtransmission process, scheduling, transport format selection, channelcoding, an inverse fast Fourier transform (IFFT) process, and aprecoding process. Furthermore, a signal of a physical downlink controlchannel, which is a downlink control channel, is also subjected totransmission processes such as channel coding and an inverse fastFourier transform.

Also, the baseband signal processing section 204 reports controlinformation for allowing each user terminal to perform radiocommunication with the radio base station 20, to the user terminalsconnected to the same cell, by a broadcast channel. The information forallowing communication in the cell includes, for example, the uplink ordownlink system bandwidth, root sequence identification information(root sequence indices) for generating random access preamble signals inthe PRACH (Physical Random Access Channel), and so on.

Baseband signals that are output from the baseband signal processingsection 204 are converted into a radio frequency band in thetransmitting/receiving sections 203. The amplifying sections 202 amplifythe radio frequency signals having been subjected to frequencyconversion, and output the results to the transmitting/receivingantennas 201.

Meanwhile, as for signals to be transmitted from user terminals to theradio base station 20 on the uplink, radio frequency signals that arereceived in the transmitting/receiving antennas 201 are amplified in theamplifying sections 202, converted into baseband signals throughfrequency conversion in the transmitting/receiving sections 203, andinput in the baseband signal processing section 204.

The baseband signal processing section 204 performs an FFT process, anIDFT process, error correction decoding, a MAC retransmission controlreceiving process, and RLC layer and PDCP layer receiving processes ofthe transmission data that is included in the baseband signals receivedon the uplink. The decoded signals are transferred to the higher stationapparatus through the transmission path interface 206.

The call processing section 205 performs call processing such as settingup and releasing communication channels, manages the state of the radiobase station 20 and manages the radio resources.

FIG. 10 is a block diagram to show a configuration of a baseband signalprocessing section in the radio base station shown in FIG. 9. Thebaseband signal processing section 204 is primarily formed with a layer1 processing section 2041, a MAC processing section 2042, an RLCprocessing section 2043, an interference level measurement section 2044,a change request signal generating section 2045, a DL/UL configurationselection section 2046 and a control signal generating section 2047.Note that the control signal generating section 2047 constitutes asignal generating section.

The layer 1 processing section 2041 mainly performs processes related tothe physical layer. For example, the layer 1 processing section 2041applies processes to signals received on the uplink, including channeldecoding, a discrete Fourier transform (DFT), frequency demapping, aninverse fast Fourier transform (IFFT), data demodulation and so on.Also, the layer 1 processing section 2041 performs processes for signalsto transmit on the downlink, including channel coding, data modulation,frequency mapping and an inverse fast Fourier transform (IFFT) and soon.

The MAC processing section 2042 performs processes for signals that arereceived on the uplink, including MAC layer retransmission control,scheduling for the uplink/downlink, transport format selection for thePUSCH/PDSCH, resource block selection for the PUSCH/PDSCH, and so on.

The RLC processing section 2043 performs, for packets that are receivedon the uplink/packets to transmit on the downlink, packet division,packet combining, RLC layer retransmission control and so on.

The interference level measurement section 2044 measures the level ofinterference from other radio base stations. For example, with theexample shown in FIG. 3, the interference level measurement section 2044in radio base station #1 measures the level of interference from anotherradio base station #2. For the measurement of the level of interference,path loss, penetration loss, antenna gain and so on may be used, and,for example, the path loss of an uplink channel (between radio basestation #1 and user terminal #1) may be measured.

Based on the interference level measured in the interference levelmeasurement section 2044, the change request signal generating section2045 generates a change request signal to request a change of the DL/ULconfiguration to the other base station. For example, with the exampleshown in FIG. 3, the change request signal generating section 2045 inradio base station #1 determines whether or not the level ofinterference from the other radio base station #2 is greater than apredetermined reference value (threshold value #1), and, if it is,generates a change request signal.

In this case, the change request signal generating section 2045 in radiobase station #1 generates a change request signal to request a change ofthe DL/UL configuration to the other radio base station #2 so that theother radio base station #2 adopts the same DL/UL configuration as inradio base station #1. Note that this change request signal includes aDL/UL configuration pattern index that is determined in advance (seeFIG. 5 to FIG. 7). For example, the change request signal generatingsection 2045 specifies a DL/UL configuration pattern index withreference to the management tables shown in FIG. 5 to FIG. 7. The changerequest signal generated in the change request signal generating section2045 is transmitted to the other radio base station #2 by wire or bywireless.

The DL/UL configuration selection section 2046 selects a DL/ULconfiguration based on the change request signal transmitted from theother radio base station. To be more specific, the DL/UL configurationselection section 2046 selects a DL/UL configuration in accordance withthe DL/UL configuration pattern index designated in the change requestsignal. For example, the DL/UL configuration selection section 2046selects DL/UL configuration with reference to the management tablesshown in FIG. 5 to FIG. 7.

Based on the DL/UL configuration selected in the DL/UL configurationselection section 2046, the control signal generating section 2047generates a control signal (downlink control channel signal: PDCCH,ePDCCH) that designates DL/UL subframe information in a radio frame. TheDL/UL subframe information includes the number and positions ofsubframes for DL and/or UL transmission. For example, when the DL/ULconfiguration of pattern index #22 in the management table shown in FIG.7 is selected, the control signal generating section 2047 generates acontrol signal which includes the number of DL subframes (three) andtheir positions (subframes #0, #3 and #4) in the DL assignment indownlink control information (DCI), and the number of UL subframes(three) and their positions (subframes #2, #5 and #6) in the UL grant.

The control signal generated in the control signal generating section2047 is output to the layer 1 processing section 2041. The layer 1processing section 2041 allocates this control signal to the downlinkcontrol channel (PDCCH, ePDCCH) of a specific subframe constituting theradio frame. For example, the layer 1 processing section 2041 allocatesa control signal that designates DL/UL subframe information to thedownlink control channel (PDCCH, ePDCCH) of subframe #0, which is at thetop of the radio frame. This control signal is transmitted to the userterminals via the transmitting/receiving section 203 as transmissionsections.

Next, an overall configuration of a user terminal according to thepresent embodiment will be described with reference to FIG. 11. As shownin FIG. 11, a user terminal 10 has transmitting/receiving antennas 101,amplifying sections 102, transmitting/receiving sections (transmittingsections/receiving sections) 103, a baseband signal processing section104, and an application section 105.

As for downlink data, radio frequency signals that are received in thetransmitting/receiving antennas 101 are amplified in the amplifyingsections 102, and converted into baseband signals through frequencyconversion in the transmitting/receiving sections 103. The basebandsignals are subjected to an FFT process, error correction decoding aretransmission control receiving process and so on in the basebandsignal processing section 104. In this downlink data, downlinktransmission data is transferred to the application section 105. Theapplication section 105 performs processes related to higher layersabove the physical layer and the MAC layer, and so on. Also, in thedownlink data, broadcast information is also transferred to theapplication section 105.

Meanwhile, uplink transmission data is input from the applicationsection 105 into the baseband signal processing section 104. Thebaseband signal processing section 104 performs a mapping process, aretransmission control (HARQ) transmission process, channel coding, aDFT process, and an IFFT process. Baseband signals that are output fromthe baseband signal processing section 104 are converted into a radiofrequency band in the transmitting/receiving sections 103. After that,the amplifying sections 102 amplify the radio frequency signals havingbeen subjected to frequency conversion, and transmit the results fromthe transmitting/receiving antennas 101.

FIG. 12 is a block diagram to show a configuration of a baseband signalprocessing section in the user terminal shown in FIG. 11. As shown inFIG. 12, the baseband signal processing section 104 is primarily formedwith a layer 1 processing section 1041, a MAC processing section 1042,an RLC processing section 1043, and a subframe information analyzingsection 1044.

The layer 1 processing section 1041 mainly performs processes related tothe physical layer. The layer 1 processing section 1041, for example,performs processes for a signal that is received on the downlink,including channel decoding, a discrete Fourier transform (DFT),frequency demapping, an inverse fast Fourier transform (IFFT), datademodulation and so on. Also, the layer 1 processing section 1041performs processes for a signal to transmit on the uplink, includingchannel coding, data modulation, frequency mapping, an inverse Fouriertransform (IFFT), and so on.

For example, when the subframe to designate DL/UL subframe informationis allocated to subframe #0 at the top of a radio frame, the layer 1processing section 1041 demodulates the control signal allocated to thedownlink control channel of this subframe. Then, the demodulation resultis output to the subframe information analyzing section 1044. Note that,when the subframe to designate DL/UL subframe information is reported bymeans of a broadcast signal, RRC signaling and so on, the control signalallocated to the downlink control channel of the reported subframe isdemodulated, and that demodulation result is output to the subframeinformation analyzing section 1044.

The MAC processing section 1042 performs, for a signal that is receivedon the downlink, MAC layer retransmission control (HARQ) and an analysisof downlink scheduling information (specifying the PDSCH transportformat, specifying the PDSCH resource blocks and so on), and so on.Also, the MAC processing section 1042 performs, for a signal to transmiton the uplink, MAC retransmission control, and an analysis of uplinkscheduling information (specifying the PUSCH transport format,specifying the PUSCH resource blocks and so on), and so on.

The RLC processing section 1043 performs, for packets received on thedownlink/packets to transmit on the uplink, packet division, packetcombining, RLC layer retransmission control, and so on.

The subframe information analyzing section 1044 analyzes the DL/ULsubframe information designated by the control signal, from thedemodulation result received as input from the layer 1 processingsection 1041. For example, the subframe information analyzing section1044 acquires the number and positions of subframes for DL transmissionfrom the DL assignment in the DCI of the control signal. Also, thesubframe information analyzing section 1044 acquires the number andpositions of subframes for UL transmission from the UL grant in the DCIof the control signal.

The DL/UL subframe information analyzed in the subframe informationanalyzing section 1044 is output to the layer 1 processing section 1041.The layer 1 processing section 1041 applies processes to the signalreceived in the subframes for DL transmission designated by the DL/ULsubframe information, including channel decoding, a discrete Fouriertransform (DFT), frequency demapping, an inverse fast Fourier transform(IFFT), data demodulation and so on. Also, the layer 1 processingsection 1041 applies processes to the signal to transmit in thesubframes for UL transmission designated by the DL/UL subframeinformation, including channel coding, data modulation, frequencymapping, an inverse Fourier transform (IFFT), and so on.

As has been described above, with the radio communication systemaccording to the present embodiment, even when different DL/ULconfigurations are adopted between neighboring radio base stations, eachradio base station 20 communicates with serving user terminals 10 byswitching the DL/UL configuration to adopt on a per radio frame basis.For example, each radio base station 20 selects a common DL/ULconfiguration with other radio base stations based on the level ofinterference received from the other radio base stations, andcommunicates with serving user terminals 10. By this means, it ispossible to reduce the impact of interference between radio basestations 20 and between user terminals 10, and reduce the deteriorationof the overall communication quality of the radio communication system.

Now, although the present invention has been described in detail withreference to the above embodiment, it should be obvious to a personskilled in the art that the present invention is by no means limited tothe embodiment described herein. The present invention can beimplemented with various corrections and in various modifications,without departing from the spirit and scope of the present inventiondefined by the recitations of the claims. Consequently, the descriptionsherein are provided only for the purpose of explaining examples, andshould by no means be construed to limit the present invention in anyway.

The disclosure of Japanese Patent Application No. 2012-127184, filed onJun. 4, 2012, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

The invention claimed is:
 1. A user terminal performing communication ina cell where a DL/UL configuration varies in time division duplexing,the user terminal comprising: a receiving section that receives acontrol signal transmitted on a downlink control channel in a specificsubframe while the user terminal is communicating in the cell; and ananalyzing section that analyzes a number and positions of subframes tobe allocated for DL and/or UL transmission among subframes constitutinga radio frame based on the control signal, wherein the receiving sectionreceives information of the specific subframe by higher layer signalingand/or by a broadcast signal.
 2. The user terminal according to claim 1,wherein the number and positions of subframes to be allocated for DLand/or UL transmission are determined in advance.
 3. A radio basestation performing communication with a user terminal by changing aDU/UL configuration in time division duplexing, the radio base stationcomprising: a signal generating section that generates a control signalthat designates a number and positions of subframes to be allocated forDL and/or UL transmission among subframes constituting a radio frame;and a transmission section that transmits the control signal to the userterminal on a downlink control channel in a specific subframe while theradio base station is communicating with the user terminal, wherein thetransmission section transmits information of the specific subframe tothe user terminal by higher layer signaling and/or by a broadcastsignal.
 4. A radio communication system comprising a user terminal thatperforms communication in a cell where DL/UL configuration varies intime division duplexing, wherein the user terminal has: a receivingsection that receives a control signal transmitted on a downlink controlchannel in a specific subframe while the user terminal is communicatingin the cell; and an analyzing section that analyzes a number andpositions of subframes to be allocated for DL and/or UL transmissionamong subframes constituting a radio frame based on the control signal,wherein the receiving section receives information of the specificsubframe by higher layer signaling and/or by a broadcast signal.
 5. Acommunication control method for a user terminal that performscommunication in a cell where DL/UL configuration varies in timedivision duplexing, the communication control method comprising:receiving a control signal transmitted on a downlink control channel ina specific subframe while the user terminal is communicating in thecell; analyzing a number and positions of subframes to be allocated forDL and/or UL transmission among subframes constituting a radio framebased on the control signal; and receiving information of the specificsubframe by higher layer signaling and/or by a broadcast signal.